Smoking high-strength cannabis may damage nerve fibres in brain

High-strength cannabis may damage nerve fibres that handle the flow of messages across the two halves of the brain, scientists claim. Brain scans of people who regularly smoked strongskunk-like cannabisrevealed subtle differences in the white matter that connects the left and right hemispheres and carries signals from one side of the brain to the other.

The changes were not seen in those who never used cannabis or smoked only the less potent forms of the drug, the researchers found.

The study is thought to be the first to look at the effects of cannabis potency on brain structure, and suggests that greater use of skunk may cause more damage to the corpus callosum, making communications across the brain’s hemispheres less efficient.

Paola Dazzan, a neurobiologist at the Institute of Psychiatry at King’s College London, said the effects appeared to be linked to the level of active ingredient, tetrahydrocannabinol (THC), in cannabis. While traditional forms of cannabis contain 2 to 4 % THC, the more potent varieties (of which there are about 100), can contain 10 to 14% THC, according to the DrugScope charity.

“If you look at the corpus callosum, what we’re seeing is a significant difference in the white matter between those who use high potency cannabis and those who never use the drug, or use the low-potency drug,” said Dazzan. The corpus callosum is rich in cannabinoid receptors, on which the THC chemical acts.

A DTI image of the corpus callosum, as seen from the side, is shown in red on and superimposed on a background MRI image of the brain.Photograph: Institute of Psychiatry

“The difference is there whether you have psychosis or not, and we think this is strictly related to the potency of the cannabis,” she added. Details of the study are reported in the journalPsychological Medicine.

The researchers used two scanning techniques, magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI), to examine the corpus callosum, the largest region of white matter, in the brains of 56 patients who had reported a first episode of psychosis, e 43 healthy volunteers from the local community.

The scans found that daily users of high-potency cannabis had a slightly greater – by about 2% – “mean diffusivity” in the corpus callosum. “That reflects a problem in the white matter that ultimately makes it less efficient,” Dazzan told the Guardian. “We don’t know exactly what it means for the person, but it suggests there is less efficient transfer of information.”

The study cannot confirm that high levels of THC in cannabis cause changes to white matter. As Dazzan notes, it is may be that people with damaged white matter are more likely to smoke skunk in the first place.

“It is possible that these people already have a different brain and they are more likely to use cannabis. But what we can say is if it’s high potency, and if you smoke frequently, your brain is different from the brain of someone who smokes normal cannabis, and from someone who doesn’t smoke cannabis at all,” she said.

But even with the uncertainty over cause and effect, she urged users and public health workers to change how they think about cannabis use. “When it comes to alcohol, we are used to thinking about how much people drink, and whether they are drinking wine, beer, or whisky. We should think of cannabis in a similar way, in terms of THC and the different contents cannabis can have, and potentially the effects on health will be different,” she said.

“As we have suggested previously, when assessing cannabis use, it is extremely important to gather information on how often and what type of cannabis is being used. These details can help quantify the risk of mental health problems and increase awareness of the type of damage these substances can do to the brain,” she added.

In February, Dazzan and others at the Institute of Psychiatryreportedthat the ready availability of skunk in south London might be behind a rise in the proportion of new cases of psychosis being attributed to cannabis.